In general, an electron gun refers to a device for making a flow of electrons converged in the form of a thin beam so as to be discharged, like an electron microscope, traveling wave tube, Braun tube, or the like.
The related art electron gun uses electromagnetic waves in order to accelerate an electron beam passing through the interior of a coupler cell. Namely, electromagnetic waves are made incident to the interior of the coupler cell through a coupling hole formed in the coupler cell. However, the symmetry of electric fields in the interior of the coupler cell is lost due to the coupling hole. The loss of the symmetry of the electric fields increases emittance of the electron beam, resulting in a degradation of quality of the electron beam.